The present invention relates to electrically joining superconductor cables. More particularly it relates to forming superconductor coils from such cables by formation of superconducting solder joints.
Many alloys of niobium and titanium have been found to be superconducting at the temperature of liquid helium. It is a common practice in the industry to embed a superconducting material within a carrier or matrix metal such as copper, copper-nickel alloy, or similar matrix conductor. In the case of a wire made of niobium-titanium alloy the alloy can be made up into the form of filaments and these filaments can be incorporated by conventional means and methods within a copper wire to form an array of the niobium-titanium filaments within the bulk of the copper carrier as a matrix. The formation of superconductors into wire form is convenient when the wire is used for a winding or coil to form a magnetic field within a loop or loops of superconducting wire. One problem in forming such loops is that at least one point, and more commonly a number of points in a superconducting coil, it requires that the wire and the elements of the wire be joined to form a continuous path through which the superconducting current can pass in generating the very high magnetic field.
While a number of successful methods have been found for forming a continuous loop by forming a joint between two wire ends, or between the two ends of a single wire, nevertheless there are needs for improved joints of superconducting wire and the present invention provides such an improved joint. The joint of this invention is illustrated by the joining together of the superconducting elements or filaments of a copper matrix wire which matrix has incorporated therein a number of filaments of niobium-titanium alloy stretched out into superconductive filaments extending through the length of the wire.
One way in which a measure can be made of the relative merits of the superconducting joint of a superconducting wire is by measuring the degree to which it remains superconducting at liquid helium temperatures at high currents and under magnetic fields such as a field of 2000 gauss. The superconducting joint of the subject invention does remain superconducting at liquid helium temperatures and with the passsage therethrough of high currents of over a hundred thousand amperes per square centimeter of superconductor when subjected to magnetic fields of at least 2000 gauss. The joints of this invention carry currents approaching the current carrying capacity of the superconductor wires and cables which are joined.
With regard to the operation of superconducting wire in its normal environment, for some applications of superconducting magnets, operation in the persistent mode is desirable. To establish the persistent mode an electromagnet wound with superconducting wire is cooled to superconducting temperature and is energized with an external direct current. After the external power supply is disconnected, the current and magnetic field stabilize and thereafter do not decay if all of the joints are superconducting.
A number of publications and patents deal with niobium-titanium superconductors and methods and means of forming joints between such superconductors.
One such publication is an article appearing in the October 1977 issue of Welding Journal starting at page 23 and entitled "Soldering of Copper-Clad Niobium-Titanium Superconductor Composite" and dealing with use of a variety of solders and fluxes. The solder joints were not superconducting. No flux was found which permitted and/or caused the solder to wet the superconducting filaments. The subject method does not employ fluxes at all.
A method of forming a superconductive butt joint between copper clad niobium-titanium superconductors by overwrapping the butt joint with smaller shunt superconductors and attaching the shunt in place by solder including a lead-bismuth solder is disclosed in U.S. Pat. No. 3,453,378. Various prior art methods of forming superconducting joints are disclosed in this patent as well as problems arising from failure of such joints.
The properties of various solders including solders containing lead and bismuth potentially useful to form superconducting joints are disclosed in the article entitled "Superconductivity Measurements in Solders Commonly Used for Low Temperature Research" appearing at page 180 of Reviews of Scientific Instruments, Vol. 40, January, 1969.
A superconductive connection involving use of solders is described in U.S. Pat. No. 3,346,351 assigned to the same assignee as the present application.
A variety of superconducting solders and their uses are described in U.S. Pat. No. 3,156,539 also assigned to the same assignee as the subject application.
Formation of a superconducting joint employing a combination of a superconducting low melting alloy containing combinations of lead-bismuth-tin and an outer crimped sleeve are taught in U.S. Pat. No. 3,449,818.
Accordingly, the use of solders or low melting combinations of molten metals in efforts to form so-called "soldered" superconducting joints is well known but varying degrees of success have been achieved in forming joints with a significant degree of superconductivity. The present invention provides solder joints having a significant degree of superconductivity and does so efficiently and economically.
The Applicant is not aware of any article, patent or other information which has taught or has made possible formation of superconducting solder joints between superconducting wire ends according to the method of this invention. Applicant is also not aware of a superconducting joint which can be constructed with the relative simplicity and reliability and at the low cost of the joints of this invention. Moreover, the Applicant is not aware of any teaching of superconductive joints formed by simple measures according to this invention employing superconducting solder, which joints remain highly conductive under magnetic fields of at least 2000 gauss. Further no low cost method is known for forming solder joints which have high quench currents of more than a hundred kiloamperes per square centimeter of semiconductor and up to levels approaching the current carrying capacity of the superconducting wire or cable which is joined.